Here and here, people attempt to answer the question: what are the chances that John McCain will die in the next four or eight years?
A quick look at mortality tables says roughly 15% in four years, 30% in eight years, which are roughly ten times the corresponding figures for Obama --- although it gets more complicated than that pretty quickly. Obama smoked for a while, McCain had cancer. Obama's parents died relatively young, which seems bad for him-- but his father from an automobile accident and his mother from ovarian cancer, which Obama himself is obviously not at risk for. McCain's mother, on the other hand, is still alive at 96. The presidency is a very stressful job -- but it comes with great health care! (I don't actually know what sort of health care the president has, but somehow I don't see doctors turning away the president for inability to pay.) And so on.
And if we're talking about the probability that a president will survive his term, we also have to think about assassination. Four out of 44 US presidents have been assassinated; what are the probabilities that either of the nominees would be assassinated? I don't even know how one would begin to assess that.
Finally, as meep points out in the first post linked above, "The central limit theorem doesn't kick in at one person". We don't get to elect a president, branch off a large number parallel worlds, and see in what proportion of those worlds he survives four or eight years. (Unless you subscribe to the many-worlds interpretation, that is.) We get one shot.
(Well, we Americans get one shot. My statistics have historically shown that about half my readers are reading from outside the US.)